Normally, the sinoatrial node (SA node) functions as the primary pacemaker of the heart. However, there are subsidiary pacemakers within the right atrium, outside of the SA node, that may function as a failsafe mechanism in the event of SA node dysfunction or may be responsible for generating atrial dysrhythmias. Although their importance is well recognized, little is known about subsidiary atrial pacemaker function. Therefore, the long range goal of this research is to elucidate the ultrastructural, electrophysiological and functional characteristics of subsidiary atrial pacemaker activity. Atrial tissue exhibiting subsidiary pacemaker activity is obtained from cats that are anesthetized with sodium pentobarbital (40 mg/kd; iv). Standard intracellular microelectrodes will be used to study the electrical activity and a photoelectric displacement transducer will be used to measure developed tension. To study the ionic currents responsible for subsidiary pacemaker activity, single pacemaker cells will be obtained by enzymatic dispersion and voltage clamped using single whole cell voltage clamp techniques. Finally, light and electron microscopy will be used to study the ultrastructural characteristics of subsidiary pacemaker fibers. Preliminary results indicate that subsidiary atrial pacemaker automaticity is determined primarily by calcium movements. Therefore, in section I of the proposal intracellular microelectrodes and tension recordings will be used to study the response of subsidiary pacemakers to specific pharmacological agents and interventions that alter calcium movements via 1) the slow inward current, 2) intracellular release from the sarcoplasmic reticulum, 3) sodium/calcium exchange, and 4) a calmodulin regulated process. In section II, enzyme dispersion will be used to isolate single pacemaker cells for single whole cell voltage clamp studies. The initial experiments will test the viability of the single cells. Current-voltage relations will be used to determine ionic conductance characteristics. Finally, experiments will be directed toward determining the ionic currents participating in subsidiary pacemaker automaticity. These studies will provide new and important information toward furthering our understanding of fundamental mechanisms of subsidiary pacemaker activity. In addition, they will provide new insight into mechanisms of atrial dysrhythmias.